Limit concurrent event sends for a room

Send federation events concurrently

URL quote path segments over federation

move handling of auto_join_rooms to RegisterHandler

Add b prefixes to some strings that are bytes in py3

Replace old-style raise with six.reraise

Return 401 for invalid access_token on logout

use python3-compatible prints

Replace some type checks with six type checks

Improve handling of SRV records for federation connections

update prometheus dashboard to use new metric names

Return a 404 rather than a 500 on rejoining empty rooms

We aren't ready to release this yet, so I'm reverting it for now.

This reverts commit d1679a4e, reversing
changes made to e089100c.

Filter ourselves out of the server list before checking for an empty remote
host list, to fix 500 error

Fixes #2141